1. Field of the Invention
The present invention relates generally to semiconductor memory devices and more particularly to semiconductor memory devices that include an assisted charge.
2. Background of the Invention
Traditional EPROM tunnel oxide (ETOX) flash memory cells and the traditional Nitrided Read Only Memory cells suffer from programming inefficiencies since large currents are generally required to perform a programming operation. ETOX flash and Nitrided Read Only Memory cells are programmed using Channel Hot Electron (CHE) injection to program the cells to a high voltage. Hot electrons are electrons that have gained very high kinetic energy after being accelerated by a strong electric field in areas of high field intensities within a semiconductor device, such as ETOX or Nitrided Read Only Memory semiconductor devices. CHE injection occurs when both the gate voltage and the drain voltage are significantly higher than the source voltage, with Vg≈Vd.
Channel carriers that travel from the source to the drain are sometimes driven towards the gate oxide even before they reach the drain because of the high gate voltage. Injected carriers that do not get trapped in the gate oxide become gate current. The injection efficiency of CHE is small, however, and programming using CHE injection requires large programming current and therefore, CHE injection is inefficient with respect to this wasted current.
Another type of memory cell, a PHINES memory cell, uses Band To Band Hot Hole (BTBHH) injection to program cells to a low voltage. Each PHINES memory cells can store 2 bits per cell. One bit can be stored on the source side of the transistor and one bit can be stored on the drain side of the transistor. In these memory cells each bit can have two states; a high current state that can represent a logic “1” and a low current state that can represent a logic “0”. Each side of the memory cell can be read by sensing the current through the cell and determining if the current is higher or lower than a threshold.
The BTB current of an erased cell is higher than the BTB current of a programmed cell. For this reason the state of each side of each cell, programmed or not programmed, can be determined by comparing the current through each side of each cell to a threshold, e.g., a gate to drain or gate to source current threshold.
In a PHINES memory device the charge accumulated on the nitride layer can be erased by a process known as Fowler-Nordheim Injection. During an erase cycle, erase voltages are applied to the source, drain, gate and body of the transistor that cause electrons to tunnel through the bottom oxide barrier of the ONO layer into the nitride layer. These electrons can compensate for the holes injected into the nitride layer during programming. The tunneling through the bottom oxide layer can occur in the presence of a high electric field, created as a result of application of the erase voltages to the transistor, and is a form of quantum mechanical tunneling.
Programming by BTBHH injection can still be too slow, and can require programming times that are too long, for certain applications.